Tbf extremism itself isn't wrong. Any perspective can be considered extreme if it is too different from the status quo. Different isn't necessarily bad.
Granted religious extremism is typically far right reactionary ideology which is bad so I'm not really defending it. However, I find that a lot of people, especially Americans, call anything that radically challenges the current system extreme and therefore bad.
The same can be said about religion. Less than 20% of Americans identify as Atheist or Agnostic, the far right extremists do not have support from 80% of the population.
You are half right but I understand where you are coming from, you see extremism as what the bigots tell you it is (feminism, LGBT+, etc). But I in fact thing that taking an idea and turning the notch to a 100 always turns it bad, for example: Feminism turns into misandry when turn to the extreme, right wing turns into facism, black right movements can turn into black power, religion turns into cults, etc.
But I agree that there are some cases in where this does not apply like gender equality (but thought I don't know how that works tho).
This is an aspect of horseshoe theory which is pretty meh tbh. Could you not say that the current status quo is extreme? It would have been considered that way by monarchists back in the day. Extremism is just radical change to the current social order which can end very well or very poorly.
Personally I think labelling ideologies as extreme is a way for those who benefit from the current social order to encourage those who don't to dismiss radical change as dangerous and destructive rather than an opportunity for growth.
Capitalism is an extreme change when compared to feudalism but it is better no?
If you're interested I'd really recommend reading blackshirts and reds by Michael Parenti pdfaudio. it's a relatively short read at only 154 pages but it really helped develop my views on this subject